Many methods for removing sulfur dioxide from a combustion exhaust gas have been heretofore proposed, including a dry method using various kinds of adsorbents, a wet method employing an alkali sulfite aqueous solution, etc. In the wet method using an alkali sulfite aqueous solution, for example, a combustion exhaust gas is introduced into the alkali sulfite aqueous solution for interacting sulfur dioxide contained in the combustion exhaust gas and the alkali sulfite to produce an alkali bisulfite, and the resultant alkali bisulfite is reacted with slaked lime or limestone to produce calcium sulfite, resulting in removal of sulfur dioxide in the form of calcium sulfite from the combustion exhaust gas. In this methods, when sodium sulfite is used as an alkali sulfite, the reactions occur as represented by the following formulae: EQU SO.sub.2 + Na.sub.2 SO.sub.3 + H.sub.2 O .fwdarw. 2 NaHSO.sub.3 ( 1) EQU 2NaHSO.sub.3 + CaCO.sub.3 .fwdarw. CaSO.sub.3 . 1/2H.sub.2 O + Na.sub.2 SO.sub.3 + CO.sub.2 + 1/2H.sub.2 O (2) EQU 2NaHSO.sub.3 + Ca(OH).sub.2 .fwdarw. CaSO.sub.3 . 1/2H.sub.2 O + Na.sub.2 SO.sub.3 + 3/2 H.sub.2 O (3)
However, when the combustion exhaust gas is introduced into the alkali sulfite aqueous solution for reaction of sulfur dioxide with the alkali sulfite, a portion of the alkali sulfite is oxidized by means of oxygen, which is also contained in the combustion exhaust gas, thereby to produce an alkali sulfate. Accordingly, a reaction solution of an alkali bisulfite which is obtained by the interaction of sulfur dioxide and the alkali sulfite also contains the alkali sulfate as an undesirable by-product.
The alkali sulfate is accumulated in the alkali bisulfite aqueous solution in a large amount, an efficiency of absorption of sulfur dioxide in the alkali suflite aqueous solution is extremely lowered. Accordingly, it is essentially required to eliminate the produced alkali sulfate from the alkali bisulfite aqueous solution. In the prior methods, sulfuric acid and calcium sulfite are added, out of the reaction system, to the alkali bisulfite aqueous solution for converting the alkali sulfate into gypsum (CaSO.sub.4.2H.sub.2 O) in accordance with the following reaction formulae (wherein sodium sulfate is used as the alkali sulfate), thus removing the alkali sulfate in the form of gypsum from the reaction solution: EQU H.sub.2 SO.sub.4 + CaSO.sub.3.1/2H.sub.2 O + 1/2H.sub.2 O = CaSO.sub.4.2H.sub.2 O + SO.sub.2 ( 4) SO.sub.2 + Na.sub.2 SO.sub.4 + CaSO.sub.3.1/2H.sub.2 O + 5/2H.sub.2 O = CaSO.sub.4.2H.sub.2 O + 2NaHSO.sub.3 ( 5)
however, the introduction of sulfuric acid and calcium sulfite out of the reaction system disadvantageously necessitates additional complicated operation.